Mass soldering system

ABSTRACT

A soldering system is described in which a fluid stream is directed onto a soldered board substantially immediately following deposition of molten solder onto the board. Preferably, but not necessarily, the fluid, which may comprise a gas or mixture of gases, is heated prior to contacting the board. The impinging fluid stream relocates solder on, and/or blasts excess solder from the bottom of the board, and any interconnections, component leads and/or component bodies carried thereon before the solder solidifies as shorts, icicles or bridges. If desired, liquid droplets such as soldering oil may be included in the fluid stream.

The present application is a Continuation of my copending applicationSer. No. 951,052, filed Oct. 12, 1978, which application in turn is acontinuation-in-part of my copending application Ser. No. 897,492, filedApr. 18, 1978, now abandoned which application in turn is acontinuation-in-part of my copending application Ser. No. 856,759, filedDec. 2, 1977, now abandoned.

The present invention relates to systems for soldering electrical andelectronic components onto substrate circuit boards, and morespecifically to an improved apparatus and method for mass solderingelectrical and electronic components by their leads, to printed circuitboards or the like.

Various soldering systems are well known in the art for mass solderingelectrical and electronic components, by their leads, onto printedcircuit boards. One technique for mass soldering components to circuitboards is that of dip soldering. With this technique, the entire side ofa circuit board containing the printing wiring, with the leads from thecomponents projecting through apertures in the board, is engaged for acertain period of time with the surface of a bath of molten solder, andthen removed. Another technique for mass soldering components ontocircuit boards is that of wave soldering. A typical prior art wavesoldering system generally comprises a container adapted to hold asupply of molten solder and a sump partially submerged in the moltensolder. The sump has an intake orifice below the surface of moltensolder, and an elongate horizontal nozzle or slot above the surface ofthe solder. A positive displacement pump is submerged in the body ofsolder and is adapted to force molten solder into the sump intakeorifice, where the molten solder then flows upward in the sump and outthe horizontal nozzle to thereby produce a smoothly rounded standingwave of molten solder above the nozzle. Other techniques for masssoldering electrical and electronic components onto printed circuitboards are well known in the art and include cascade soldering, jetsoldering and drag soldering. So-called "leadless" components such asflat packs can also be mass soldered to circuit boards by fixing thecomponents to the bottom of a board, e.g. as by fixturing or with anadhesive, and then engaging the bottom of the board and the componentswith molten solder. While known mass soldering systems have providedsubstantial manufacturing economy to the electronics industry and thusachieved substantial commercial use, the deposition of excess solder onthe board circuits, connections and leads has been a continual problem.Deposition of excess solder may result in formation of shorts, iciclesand/or bridges, and will increase solder consumption and finished boardweight. Moreover, current trends in the electronics industry torelatively high density electronic assemblies has increased the problemof solder shorts, icicling and bridging.

The prior art has devised various techniques to solve the problems ofsolder shorts, icicling and bridging. For example, for wave soldering,one technique which has become virtually universally adopted by theindustry is to incline the travel path of the circuit boards through thesolder wave, i.e. from the horizontal, to increase the exit anglebetween a board being soldered and the solder wave. The art has alsodevised various wave geometries for further increasing the exit angleand/or changing the point where a circuit board exits the wave. Anothersystem for reducing the incidence of solder shorts, icicling andbridging, which has achieved substantial commercial acceptance, is tointimately mix soldering oil in the solder wave in accordance with theteachings of Walker et al. U.S. Pat. No. 3,058,441. While such systemshave been found to reduce substantially the incidence of solder shorts,bridging and/or icicling, such systems have not entirely eliminatedsolder shorts, bridges and icicling, particularly in cases whererelatively high density electronic assemblies and/or relatively longlead components are being soldered to circuit boards.

It is thus a primary object of the present invention to provide a masssoldering system, i.e. apparatus and process, which overcomes theaforesaid problems of the prior art.

Another object of the present invention is to provide an improvedapparatus and process for mass soldering in which the problems of soldershorts, icicling and/or bridging are reduced.

A more specific object is to provide an apparatus and process for masssoldering relatively high density circuit board assemblies.

Still other objects will in part appear obvious and in part will appearhereinafter.

The invention accordingly comprises the processes involving the severalsteps and relative order of one or more of such steps with respect toeach other and the apparatus possessing the features, properties andrelations of elements which are exemplified in the following detaileddescription and the scope of the application of which will be indicatedin the claims.

The instant invention overcomes the foregoing and other problems byproviding a method and apparatus for removing excess solder from masssoldered boards before the solder solidifies as shorts, icicles orbridges. The method comprises the step of directing a fluid stream ontothe bottom of a soldered board substantially immediately following massdeposition of the molten solder onto the board. The fluid streamimpinges onto the bottom of the board, and relocates excess solder on,and/or blasts excess solder from the bottom of the board and anyinterconnections, component leads and/or component bodies carriedthereon before the solder solidifies as shorts, icicles or bridges. Thefluid may comprise a gas such as an inert gas, or a mixture of gasessuch as air. If desired, liquid droplets such as soldering oil may beincluded in the fluid stream. Preferably, but not necessarily, the fluidstream is pre-heated prior to contacting the board.

In the following description the term "solder removal" is used to denoteactual displacement of solder from a circuit board, and anyinterconnections, component leads and/or component bodies thereon, aswell as relocation of solder on a circuit board, and anyinterconnections, component leads and/or component bodies thereon.

The instant method is implemented with a mass soldering apparatus whichessentially comprises a soldering station in which a quantity of moltensolder can be mass deposited onto the bottom of the board to remove aportion of the solder deposited on the bottom of the board, and anyinterconnections, component leads and/or component bodies carriedthereon before the solder cools below liquidus temperature.

For a fuller understanding of the objects of the present invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIG. 1 is a side elevational view, diagrammatically illustrating a masssoldering system according to the present invention;

FIG. 2 is a side elevational view, partly in section, of the solderingapparatus portion of the soldering system of FIG. 1;

FIG. 3 is a top plan view of the soldering apparatus portion of FIG. 2;

FIG. 4 is a schematic diagram of the electrical and pneumatic controlmeans of the same apparatus;

FIG. 5 is an enlarged, side elevational view in section, of a portion ofthe apparatus of FIG. 2, showing a circuit board assembly at anintermediate stage in the process of the present invention, and showinghow excess solder is removed in accordance with the present invention;

FIG. 6 is a view similar to FIG. 5 illustrating an alternative solderingapparatus according to the present invention;

FIGS. 7 and 8 are top plan and cross-sectional views, respectivelyshowing one form of fluid stream directing nozzle structure useful inthe soldering apparatus of the present invention;

FIG. 9 is a side elevational view in perspective showing another andpreferred form of fluid stream directing nozzle structure useful in thesoldering apparatus of the present invention; and

FIG. 10 is a side view in cross-section of the nozzle structure of FIG.9.

In the following detailed description of the present invention, the term"component" refers to leadless components as well as components havingconventional metallic conductors or leads. The term "component lead"refers to that part of metallic conductor of an electrical or electroniccomponent that is joined to the printed circuit board pattern, i.e. thecomponent leads, terminals, lugs, pins, etc. The term "land" as usedherein refers to that part of the metallic pattern on the printedcircuit board to which a component or component lead is joined bysolder. The term "fluid" is to be understood to refer to a gas ormixture of gases. If desired, the gas or mixture of gases may alsocontain liquid droplets dispersed therein. The term "mass soldering" asused herein is intended to refer to any of the several solderingtechniques known in the art in which liquid solder is applied to acircuit board from a reservoir of liquid solder, including, by way ofexample but not limitation: wave soldering, touch or dip soldering, potsoldering, jet soldering, cascade soldering and drag soldering.

Referring now to the accompanying drawings, a preferred embodiment ofthe present invention will be described in combination with a wavesoldering system. Referring first to FIG. 1, a printed circuit board 20is loaded at an insertion station 22, with a plurality of electrical orelectronic components 24 at predetermined positions on the board. Theboard comprises an insulated wiring board having one or more printedmetallic conductors on the board underside, and a plurality of apertures25 which extend through the board. Components 24 are loaded onto the topside of the board with their leads 26 protruding downward through theboard apertures 25 in juxtaposition to the circuit lands to which theyare to be joined. The components may be inserted in the board by anymethod known in the art which may include manual assembly,semi-automatic, or automatic assembly which may comprise single-stationof multiple-station pantagraph or numerically controlled machines all ofwhich are well known in the art and need not be described further.

The next step involves treating the surfaces to be soldered with aso-called flux at a fluxing station 30. The flux may be any flux wellknown in the art and may include, for example, a water-white rosin flux,an activated rosin flux or a water soluble flux. The flux may be appliedin fluxing station 30 by any manner well known in the art, for example,by spraying, foaming, brushing, or from a flux wave.

The board is then passed from fluxing station 30 to a preheating station32 where the board is preheated to mobilize the flux and also drive offthe bulk of the flux solvent to form an active flux layer on the boardand leads. Preheating station 32 may comprise infrared or convectionheaters or a combination of infrared and convection heaters as are wellknown in the art. Preferably, but not necessarily, preheating station 32is extended as compared with conventional preheating stations, and/orpreheating station 32 may be operated at higher than normal temperaturesso that the board 20 is heated to higher than normal top sidetemperatures. Thus board 20 will be preheated to a minimum top sidetemperature of about 66° C.; preferably however, the board will bepreheated to a top side temperature in the range of about 100° C.-125°C. or higher. The purpose of preheating the board to higher than normaltop side temperatures is to increase the time the solder on the boardremains molten after the board emerges from the solder wave. The reasonfor this will become clear from the description following.

The fluxed preheated board is then passed to a mass wave solderingstation 36. Referring also to FIGS. 2 and 3 the wave soldering stationincludes a container of conventional design, indicated generally at 40,for holding a supply of molten solder 42. Conventional heating means(not shown) may be secured to the bottom and/or side walls of container40 or immersed in the solder to heat and maintain the supply of solder42 in molten condition.

A sump and nozzle assembly indicated generally at 44 is disposedinteriorly of container 40. The sump and nozzle assembly 44 is ofconventional design and typically comprises a rounded bottom wall 46, apair of substantially vertically opposed end walls 48 and 50, and a pairof inclined side walls 52 and 54. The upper ends of end walls 48 and 50and side walls 52 and 54 are spaced from one another to form a narrowelongated rectangular nozzle or slot 56 which extends above the moltensolder level in container 40 for a suitable distance, e.g. one inchabove the molten solder level.

Preferably, the sump also includes a pair of adjustable sluice plates58A, B spaced from the sump side walls 52 and 54 for controlling solderoverflow from the nozzle 56, e.g. in accordance with the teachings ofU.S. Pat. No. 3,398,873 to Kenneth G. Boynton. Completing the solderingstation is a variable speed pump (not shown) which communicates throughan intake orifice 59 in the lower end of sump and nozzle assembly 44 forpumping solder into the sump where it then rises and overflows thenozzle 56 as a standing solder wave.

An important feature and critical requirement of the present inventionis the ability to relocate excess solder on, and/or remove excess solderfrom the bottom of the circuit board, and from any interconnections,component leads and/or component bodies carried thereon before thesolder can solidify as shorts, icicles and/or bridges. This isaccomplished by treating the soldered circuit board and dependingcomponent leads at an excess solder removal station 60. Excess solderremoval station 60 follows soldering station 36 immediately in-line andis designed to relocate or blow off excess solder from the boardunderside before the solder solidifies as shorts, icicles and/orbridges. Solder removal station 60 comprises one or more fluid jets,fluid knives, slots, nozzles or the like indicated generally at 62, fromwhich a fluid stream can be directed onto the underside of the board. Ifdesired, a baffle plate 64 may be disposed under the path of travel ofboards 20 at an angle of approximately 45° from the horizontal andserves as a deflector for the fluid stream issuing from nozzle 62.Desirably, but not necessarily, the fluid is pre-heated prior toimpinging on the board. Fluid flow rate, fluid pressure, and fluidtemperature and the time elapsed between circuit board emersion from thesolder wave and beginning of contact by the fluid stream may vary widelydepending on the board temperature, ambient temperature, melting pointof the solder, specific heat of fluid and heat transfer coefficient offluid to the board, board size and shape, component density, amount ofsolder deposited and to be removed, conveyor speed, and distance betweenthe soldering station and the excess solder removal station. Preferablynozzle 62 is disposed proximate the path of travel of the boards. Nozzle62 of course must be spaced sufficiently below the path of travel of theboards to permit clearance of the longest depending lead, etc. Inert gasmay be used as the fluid, but preferably the impinging fluid comprisesair. The fluid may be at ambient temperature preferably however, thefluid is pre-heated to a temperature in the range of about 93° C. to350° C., preferably about 290° C. to 300° C. (measured at the outlet ofnozzle 62). For 63/37 solder alloy, the preferred fluid preheattemperature is about 290° C. (measured at the outlet of nozzle 62).

The fluid stream impinging on still molten solder contained on theunderside of the circuit board, the interconnections, and the componentleads and/or bodies relocates excess solder on, and/or blasts excesssolder from the underside of the board, interconnections, leads, andbodies, and in doing so also minimizes the possibility of solderbridging or icicling or short formation upon solidification.

FIG. 4 illustrates a preferred form of electrical and pneumatic controlmeans in accordance with the present invention and is particularlyadapted to the use of hot air as the fluid stream in accordance with thepresent invention. Referring to FIG. 4, nozzle 62 is connected viasupply line 66 to one side of solenoid actuated valve 68. Valve 68 isconnected via a line 70 to the outlet of a heater 72 which is adapted toheat air (or other gas) to a desired elevated temperature. Valve 68 isactuated by a photoelectric cell 74 connected through suitable relaymeans 76 to provide a blast of heated air when a printed circuit board20 passes through the solder wave and the board leading edge interruptsa light beam from light sources 78 disposed above the path of travel ofthe printed circuit board. The flow of heated air continues until thetrailing edge of the printed circuit board passes and permits the beamof light from light source 78 to fall once again upon the photoelectriccell 74, at which time the valve 68 is closed or nearly closed.Preferably valve 68 is prevented from closing fully, i.e. so as to allowat least a small flow of heated air through the nozzle so that thenozzle will be maintained at the desired elevated temperature and thuseliminate thermal lag.

Completing the soldering system is a circuit board conveyor 80 ofconventional construction. The latter comprises a pair of spacedconveyor rails 82 and 84 and suitable drive means (not shown) forcarrying a circuit board from the inserting station 22 through thefluxing station 30, wave soldering station 36 and excess solder removalstation 60.

FIG. 5 shows a printed circuit board in the excess solder removalstation 60 and illustrates how the hot fluid flow removes molten solder86 from a circuit board, interconnections and component leads inaccordance with the present invention.

One skilled in the art will be able to determine experimentally thepreferred operating parameters for achieving icicle-, bridge- andshort-free mass soldering for the particular board being soldered.

For example, to eliminate solder icicles, bridges and shorts from acircuit board which measures 6.35 and 15.24 cm. and includes a netcircuit plan area of approximately 96.75 sq. cm., 150 plated throughapertures of 0.0762 cm. diameter, and including 150 component leads of0.0508 cm. diameter (50% steel leads, 50% copper leads), solder alloy of63/37, solder bath temperature 252° C., preheat temperature of about125° C. (top side) conveyor speed of 0.018 m/sec., and employing hot air(290° C., measured at the outlet of nozzle 62), at a Pitot tube pressure(measured approximately 0.6 cm from the outlet of nozzle 62) of about22.5 mm Hg, and a fluid velocity (measured at the outlet of nozzle 62)of about 76 m/sec., as the impinging fluid, the distance between thepoint where the circuit board emerges from the solder wave and begins topass the hot air stream should be in the range of about 19 to 25 cm. Oneskilled in the art will recognize that the various aforesaid parametersare interrelated and may be varied to achieve the foregoing objects ofthe present invention.

Moreover, the present invention has a number of advantages. For one,contacting the underside of a circuit board with a fluid stream inaccordance with the present invention has been found to level the soldercoating on the board conductors. Moreover, eyelets and unloaded platedthrough holes may be cleared by the fluid stream. Another advantage isthat a soldered board may emerge from the system somewhat cooler ascompared with conventional mass soldering systems. This latter advantageresults primarily from the removal of excess solder which is a majorheat sink on conventionally soldered boards. Moreover, the fluid stream,if cooler than the solder on the board, may further speed cooling.Cooling the board facilitates handling of the board subsequent tosoldering and also may result in reduced incidence of pad lift insolder-cut-solder systems. Cooling the board also may result inproduction of solder joints of finer grain.

Various changes may be made in the foregoing invention without departingfrom the inventive features described herein. For example, one or morebanks of heaters, similar to construction to the preheaters may beincorporated into the excess solder removal station 60 to extend thetime the solder remains molten on the board. Moreover, where the fluidcomprises a gas or gas mixture, a minor amount, e.g. up to about 20% byweight of liquid droplets may be dispersed in the gas stream, forexample, by aspirating the liquid into the gas stream from one or moreaspirator nozzles 98 as shown in FIG. 6. Aspirator nozzles 98 areconnected through conduit means 100 to a supply 102 of the liquid to bedispersed into the gas stream. Obviously, the liquid droplets could beinjected into the gas stream using one or more atomizing nozzles. Theliquid should comprise a material which is compatible with the solderingoperation. By way of example, the liquid may comprise a conventionalsoldering oil or mixture of oils. The liquid droplets increase the fluidflow mass of the gas stream and thus in turn may result in an increasedrate of solder removal from the board and leads. Also, employing asoldering oil as the liquid may facilitate post-soldering clean-up andmay also produce shinier solder joints. If desired, conventional wettingagents and/or fluxing agents may also be dispersed in the fluid stream.Also, one or more heater elements may be incorporated integrally withnozzles 62, e.g. as shown in FIGS. 7 to 10 to supplement and/or in placeof heater 72.

Referring in particular to FIGS. 7 and 8, the illustrated heater/nozzlecombination comprises a manifold in the form of a flat, elongatetrapezoidal hollow chamber indicated generally at 104. An intake orifice106 is formed in the short side wall 108 of manifold 104, and is coupledthrough conduit means (not shown) to a source of pressurized air (notshown). One of the long walls 110 of manifold 104 is tapered to form anelongate knife-edged orifice 112. The latter defines the nozzle outletfor directing a fluid stream onto a circuit board in accordance with thepresent invention. Manifold 104 is formed of a heat resistant materialsuch as welded steel plate. Mounted in the interior of manifold 104 is aheating means such as one or more electrical resistance heater elements114. Electrical resistance heater elements 114 are known in the art andare available commercially. If desired, the manifold walls may bethermally insulated in known manner. In operation the fluid stream isheated by passing over the resistance heaters 114.

Another and preferred embodiment of heater/nozzle combination is shownin FIGS. 9 and 10. In the illustrated preferred embodiment the nozzlecomprises a generally rectangular block 116. Block 116 is formed of aheat resistant material such as steel. A plurality of blind holes 118are formed in one side wall 120 of block 116 and accommodate a likenumber of cartridge heaters 122. Cartridge heaters are well known in theart and are available commercially. A hollow chamber 124 is formed inthe interior of block 116, and an inlet orifice 125 is formed in one endwall of block 116 and connects hollow chamber 124, through conduit means(not shown) to a source of pressurized air (not shown). The nozzleoutlet orifice is formed in the side wall 126 opposite the wall 120 inwhich cartridge heaters 122 are mounted. As seen in FIG. 10 wall 126 isbevelled so as to form a pair of parallel end edge surfaces 128 formedat the intersection of inclined surfaces 130 with elongate passageway132. The latter communicates with hollow chamber 124. Edge surfaces 128should be substantially flush with one another, and may be substantiallyknife edges as shown or may have a more substantial width as would beproduced by truncating the end of the nozzle outlet along a plane asrepresented by dotted line 134. An interesting feature and advantage oftruncating the nozzle outlet to give it a substantial width is that theworking gas stream emerging from the nozzle is found to be especiallysmooth. Also, with a nozzle outlet of this type secondary air streamshave been observed to form to either side of the working gas stream.These secondary air streams provide fluid insulating sheets for theworking gas stream and may also directly assist in relocating orremoving solder.

While the solder removal station in accordance with the presentinvention has been illustrated in conjunction with a mass wave solderingsystem one skilled in the art will recognize that similar advantages maybe achieved by employing a solder removal station in conjunction withother types of mass soldering systems such as dip, cascade, jet and dragsoldering systems. Still other changes will be obvious to one skilled inthe art. Accordingly, it is intended that all matter contained in theabove description or shown in the accompanying drawings shall beinterpreted in an illustrative and not in a limiting sense.

What is claimed is:
 1. A method of mass joining with solder printedcircuit boards having mounted thereon components with leads protrudingdownward through holes in said board, said method comprising the stepsof:depositing a quantity of molten solder onto the underside of saidboard and said downward protruding leads by passing the bottom surfaceof the board over a solder wave and substantially immediately followingsaid depositing impinging a stream of heated air directly into themolten solder deposited on the underside of said board and saidprotruding leads by means of an air knife blast directed towards thesolder wave and impinging on the bottom surface of the boardsufficiently soon after it leaves the solder wave so that the solder isstill molten, the air knife blast having sufficient velocity to removeessentially all bridges and shorts but not sufficient velocity toadversely affect properly wetted joints.
 2. A method according to claim1, wherein said air stream is heated to a temperature in the range of93° C. to 350° C.
 3. A method according to claim 2, wherein said airstream is heated to a temperature in the range of about 290° C. to 300°C.
 4. A method according to claim 1, further including the step ofdispersing liquid droplets in said air stream, and directing theresultant dispersion onto said board and leads.
 5. A method according toclaim 4, wherein said liquid droplets comprise soldering oil.
 6. Amethod according to claim 4, wherein said liquid droplets comprise awetting agent.
 7. A method according to claim 4, wherein said liquiddroplets comprise a flux agent.
 8. A method of reducing the incidence ofsolder shorts, icicling and/or bridging in a mass soldering process inwhich a quantity of molten solder is deposited onto the underside of acomponent-carrying printed circuit board and onto those parts of saidcomponents which extend below said board underside and at leastpartially fill with molten solder any holes in said board, said methodcomprising the steps of:depositing a quantity of molten solder onto theunderside of said board and said component parts by passing the bottomsurface of said board over a solder wave and substantially immediatelyfollowing said depositing impinging a stream of heated air directly ontothe molten solder deposited on the underside of said board and saidcomponent parts by means of an air knife blast directed towards thesolder wave and impinging on the bottom surface of said boardsufficiently soon after it leaves the solder wave so that the solderthereon is still molten, the air blast having sufficiently low velocityto leave a solder coating in said holes and sufficiently high velocityto remove solder shorts, bridges and icicles remaining on said board asit leaves the solder wave, the air blast increasing forces tending toremove the solder shorts, bridges and icicles.
 9. A method according toclaim 8 wherein said air stream is heated in a temperature in the rangeof 93° C. to 350° C.
 10. A method according to claim 9, wherein said airstream is heated to a temperature in the range of about 290° C. to 300°C.
 11. A method according to claim 8, further including the step ofdispersing liquid droplets in said air stream, and directing theresultant dispersion onto said board and leads.
 12. A method accordingto claim 11, wherein said liquid droplets comprise soldering oil.
 13. Amethod according to claim 11, wherein said liquid droplets comprise awetting agent.
 14. A method according to claim 11, wherein said liquiddroplets comprise a flux agent.
 15. Apparatus for mass joining withsolder electrical and electronic components assembled in a circuit boardwherein said components have leads which protrude downward throughapertures in the board, and comprising in combination:a wave solderingstation adapted to provide a wave of molten solder; means fortransporting said circuit board across said solder whereby a quantity ofmolten solder may be deposited onto said circuit board underside andsaid protruding leads; and an excess solder removal station adjacentsaid wave soldering station, said excess solder removal stationcomprising at least one hot air knife disposed below the travel path ofsaid board and adapted to direct a blast of heated air directly onto themolten solder deposited on said board underside, a source of pressurizedair, means connecting said source and said hot air knife, and means forheating said air knife blast prior to directing the latter onto saidboard underside, the air knife blast being directed towards the solderwave and impinging on the bottom surface of the board sufficiently soonafter it leaves the solder wave so that the solder is still molten, theair knife blast having sufficient velocity to remove essentially allbridges and shorts but not sufficient velocity to adversely affectproperly wetted joints.
 16. Apparatus according to claim 15, and furtherincluding means for introducing liquid droplets into said air stream.17. Apparatus according to claim 16, including a source of said liquid,and wherein said means for introducing comprises at least one aspiratorconnected to said source of said liquid.
 18. Apparatus according toclaim 16, including a source of said liquid, and wherein said means forintroducing comprises at least one atomizing nozzle connected to saidsource of said liquid.
 19. The apparatus of claim 15, wherein said airknife is provided with valve means to give a continuous small flow ofair through the knife; and means for detecting the presence of a boardover the solder wave to open the valve and provide a full air blast tothe board as it leaves the wave.